Modern industrial plants, especially those for processing chemicals, petrochemicals, and the like, have large numbers of interconnected pipes. These pipes communicate media such as fluidal liquids and gases for processing in the plants. The media carried in the processing plant pipes are typically at high temperatures and/or high pressures.
Adjacent longitudinally aligned sections of pipe connect together at junctions with bolts that extend through aligned bores in the facing flanges at the ends of the respective pipes. A resilient material, or gasket, is typically disposed between the parallel mating faces of the flanges of the pipes to be joined together. The gasket seals the interconnection between the adjacent pipes to restrict media leakage from between the flanges forming the connection between the pipes. Generally the flanges interconnect together by a plurality of bolts that pass through bores in the flanges. The bolts are secured by nuts, in order to join the pipes tightly together. The number and spacing of bolts and the geometric arrangement of the bolts around the flanges depends primarily on the diameter of the pipes and the pressure of the media flowing through the pipes and the flange connection.
As described above, gasket materials are used to seal the connection between the two flanges. Gaskets effect seals by deforming and filling the surface irregularities in the faces of the flanges. The gasket is compressed between the parallel faces of the flanges. The internal pressure of the media flowing through the flange joint attempts to blow out the gasket from between the flange faces. Hydrostatic end force, which originates with the pressure of the confined fluid, also attempts to separate the flange faces. The torqued bolts and nuts securing the flanges together resist these forces while holding the flanges together with the gasket compressed between the faces of the flanges for sealing the connection from leaks.
There are a number of known types of gaskets for sealing the flange connection. These gasket types include o-rings, plate-like gaskets or spiral wound, and gaskets cut from sheets. The pre-cut gaskets are particularly useful for forming gaskets of irregular sizes and during emergency situations requiring a temporary gasket.
Maintenance personnel use sheet gasket material to cut a gasket to fit a particular application. For many years, the primary type of sheet packing was asbestos fiber sheet having elastomeric or rubber binder. Due to environmental concerns, asbestos has generally been removed from the market and the packing industry has sought suitable substitute materials which take into account the pressure, temperature, and chemical requirements of gasket applications. One known sheet packing is graphite paper or sheet graphite. Graphite sheet is formed from intercalated flake graphite which is expanded into worms, or vermiform, and then calendared into thin, usually high density sheets of graphite. Intercalated flake graphite is formed by treating natural or synthetic graphite flake with an intercalating agent such as fuming nitric acid, fuming sulfuric acid, or mixtures of concentrated nitric acid and sulfuric acid. The intercalated flake graphite is then expanded at high temperatures to form a low-density, worm-like form of particulate graphite having typically an eighty-fold increase in size over the flake raw material.
The production of intercalated flake graphite is an intermediate step in the production of expanded intercalated graphite as described in U.S. Pat. No. 3,404,061. Expanded intercalated graphite particles have thin structural walls and are light-weight, puffy, airy, and elongated worms or vermiform. For some gasket applications, the calendared graphite sheet overlays a metal blank having an annular opening which aligns with the pipes to be sealed. The graphite sheet is coated with an adhesive for adhering the sheet to the metal blank. The sheet is cut to form the opening through the gasket. For other applications, the calendared graphite sheet is cut to the particular size and shape of the flanges to be sealed. As discussed above, the flanges are bolted together and compress the gasket between the faces of the aligned flanges for sealing the joint.
Sealability is an important physical characteristic indicative of whether a gasket material will function properly. The American Society of Testing and Materials provides a test designated F37 for evaluating the fluid sealing properties of gasket materials. When ASTM F37 is used as an acceptance test, generally sealability is evaluated with test conditions agreed upon by the manufacturer of the gasket material and the customer planning to use the gasket material in a sealing application. These test conditions include the fluid to be sealed, the internal pressure of the fluid, and the flange load. Gaskets are conventionally tested for comparison purposes with nitrogen gas at an internal pressure of pounds per square inch and a flange load of 3,000 pounds per square inch, pursuant to ASTM F37. Measurement of the Leakage rates at these conditions allow comparing one gasket material with another. The report on ASTM designation F37 explains that the question is not whether a particular gasket material allows leakage, but rather how much leakage occurs with a given set of conditions of time, temperature, and pressure. The leakage measured comes either through the gasket, between the gasket and the flange faces, or both. The ASTM report states that experience shows that in most cases, the leakage measured is a result of leakage through the gasket.
While gaskets formed from calendared graphite sheet material generally perform satisfactorily for sealing in high-temperature and high-pressure environments, the graphite sheet material has drawbacks that limit its use in large industrial facilities. One problem with flexible graphite is vibration which results in the gaskets metal substrate breaking due to metal fatigue from harmonic resonance should the gasket have uneven loads applied on its installation. A system responds when it is encouraged to vibrate at a natural frequency. If a pulsating excitation is applied with a natural frequency, then a violent motion may be expected. This phenomenon is called resonant vibration. A forced vibration becomes significant only if resonance occurs. However, resonant vibration can lead to destruction of the mechanical system being vibrated. By way of example, it is noted that resonant vibrations has caused destruction of bridges and other structures. Indeed, vibrations are destructive to sealed flange connections as well. Vibration relates to gasket breakage in the following manners. Vibration can loosen even the tightest nut, sometimes in a matter of seconds. Loosened nuts lead to loss of seal on the connection and spillage of the media communicated through the pipe. Under high pressure and flow rate, the loss of seal may be destructive and dangerous to persons and equipment in the area. Further, vibration is associated with fluctuating stresses and, during sufficient violent motion, these fluctuations may become large enough for breakage of a gasket to occur. Catastrophic breakage of a sealing gasket as a result of resonant vibration can be quick and cleanly made, and these characteristics are aspects of fatigue failure. The flow of the media in the piping can act as an exciting source for the vibration. The flange connection located in proximity of a vibration generating source, some even hard to distinguish and identify for remedial measures, can cause resonant vibration to happen.
Efforts have been made to address this problem with damping or other methods, including decreasing the natural frequency by decreasing the stiffness/mass ratio; increasing friction and/or contact surfaces that rub together during vibration, such as with the addition of inner or outer guide-rings; use of high damping material; sandwich construction of gaskets; adding vibration absorbers, such as polymeric material packing, if possible; and use of fasteners such slotted, castellated nuts or lock-nut. While these methods and structures have been used, they have riot resolved the problem presented by vibration stress on gasket-sealed flange connections.
In addition to vibration destruction of gaskets, flanged connections have other problems. In some instances, it is difficult to verify by visual inspection of the connection that the gasket remains properly seated for sealing the connection. Also, the type and character of the gasket is not readily determinable. In such circumstances, the connection often has to be broken apart with removal of the fastening bolts to verify proper gasket sealing and placement. This action however necessitates the installation of a new gasket, which is time consuming and may incur significant expenses such as downtime for the process communicating media through the connection as well as replacement gasket and materials expenses.
Accordingly, there is a need in the art for an improved gasket that seals flanged connections while changing a natural frequency of the gasket to avoid resonance in the flange connection, particularly resulting from uneven flange loading, and with increased visual verification of the seal, together with an apparatus and method of manufacturing improved flange connection gaskets. It is to such that the present invention is directed.